Arsenic is a naturally occurring element that is widely distributed in the earth's crust. Exposure to arsenic can cause a variety of adverse health effects, including dermal changes, respiratory, cardiovascular, gastrointestinal, genotoxic, mutagenic and carcinogenic effects. Arsenic contamination of drinking water has been widely reported in Argentina, Bangladesh, Cambodia, Chile, China, Ghana, Hungary, Inner Mongolia, Mexico, Nepal, New Zealand, Philippines, Taiwan, the United States and Vietnam.
The World Health Organization's arsenic guideline value for drinking water is 10 ppb. Many detection methods have been developed to detect such levels of arsenic. The most reliable techniques are more suitable for laboratory conditions only and are generally time consuming. In contrast, electrochemical methods provide accurate measurements of low concentrations of metal ions at the ppb levels with rapid analysis times and low cost instrumentation. Arsenic can be detected using electrochemical stripping voltammetry methods. Forsberg et al [(1975) Anal Chem 47:1586] describe the determination of arsenic by anodic stripping voltammetry (ASV) and differential pulse anodic stripping voltammetry (DPSAV) at various electrode materials (Hg, Pt and Au). Simm et al [(2004) Anal Chem 76:5051] disclose the sonoelectroanalytical arsenic detection on a gold electrode. Most recently, using anodic stripping voltammetry at a gold nanoparticle modified glassy carbon (GC) electrode, a limit of detection (LOD) of 0.0096 ppb was obtained [Dai et al., (2004) Anal Chem 76:5924].
Although gold and platinum electrodes can be used to determine As (III) levels via anodic stripping voltammetry, the use of such electrodes is hindered by the interference of other metals, for example lead, copper, zinc, iron, antimony, silver, selenium, bismuth and mercury. Among these metals, copper (in the form of Cu (II)) is by far the most common and ubiquitous in water systems; indeed copper is found in relatively high levels in the world's water supplies. Thus, Cu (II) potentially presents a serious interference problem in arsenic detection, especially if conventional stripping voltammetry is employed. At a gold macroelectrode, Cu codeposits with As during the pre-deposition step and forms the intermetallic compound Cu3As2 as well as bulk copper metal. The stripping peak of Cu is seen at a similar but slightly more positive potential than the As stripping peak. If the concentration of Cu (II) is sufficiently high then the stripping peak of Cu (II) partly masks the As (III) signal. A separate stripping peak, which is close to the arsenic stripping peak, due to Cu3As2 is also observable so that three maxima are possible in the stripping curve of arsenic when Cu (II) is present (see FIG. 1 herein).
The oxidation of As (III) to As (V) can be detected using a platinum electrode. Lown et al [(1980) Anal Chim Acta 116:41] have detected arsenic (III) by oxidation at a platinum wire electrode in perchloric acid solution using a flow-through cell. Williams et al [(1992) Anal Chem 64:1785] performed pulsed voltammetric detection of arsenic (III) using a rotating disk platinum electrode. In both these cases, significant levels of arsenic were employed.
There remains a need for an electrochemical method of detecting arsenic which is unperturbed by the interference of other metals, in particular copper. There also remains a need for electrode materials which have a desirable limit of detection (LOD) for arsenic, and which can be fabricated readily and at low cost.
Cui et al [(2005), J Electroanal Chem, 577:295] describe a carbon electrode modified with nanoparticulate platinum. This electrode is described as being useful in the electrocatalytic reduction of oxygen.
Indium tin oxide (ITO) films are of great interest because of their high optical transparency and good electrical conductivity. Gold nanoparticle-modified ITO glass electrodes have been used in the electrochemical detection of nitric oxide, oxygen, hydrogen peroxide and carbon monoxide. Gold nanoparticles have been grown onto ITO coated glass by seed-mediated growth (Zhang et al, Anal. Chim. Acta., 2005, 540, 299; Zhang et al, Electrochem. Commu., 2004, 6, 683; and Zhang et al., Electroanal. Chem., 2005, 577, 273) or by assembling with polymer (Huang et at, Anal. Chim. Acta., 2005, 535, 15; Patolsky et al, J. Electroanal. Chem., 1999, 479, 69; and Jaramillo et al, J. Am. Chem. Soc., 2003, 125, 7148.26-28).